Described accurately / perfectly below. It is used to balance all of the pec work that is done and also many German coaches believe you can actively place the discus behind you as opposed to it being ‘left behind’ passively.
What are your thoughts on this? I was originally taught to be passive but the arm always snuck up. Then I was told to actively hold the arm back, as if to squeeze a popsicle stick between your lat and tricep and had much better results.